1. Field of the Invention
The present invention relates to a voltage regulator meant to be connected in parallel to a load for maintaining a supply voltage of the load at a predetermined value even in case of variation of the current surged or drawn by the load. The present invention more specifically relates to a regulator which supplies a reference voltage independent from the temperature and generated by a circuit in bipolar technology using the forbidden band of silicon.
2. Discussion of the Related Art
FIGS. 1 and 2 show two conventional examples of association of such a regulator with a load.
The regulator (REG) 1 generally includes two supply terminals, respectively a positive terminal K and a negative terminal A, and a terminal Vref providing a reference voltage stable in temperature. For a positive voltage regulator, terminal K is connected to a supply terminal V+ via a resistor R1 and terminal A is connected to the ground. The load (Q) 2 is connected to terminals K and A.
The function of regulator 1 is to vary the current taken by the regulator from node K to maintain voltage V.sub.KA constant during variations of the current surged or drawn by load 2. The current 1 taken from the supply is substantially constant, only its distribution between load 2 and regulator 1 is modified.
In the example of FIG. 1, terminal Vref is connected to terminal K, load 2 being meant to be supplied under a voltage V.sub.KA corresponding to voltage Vref.
In the example of FIG. 2, a resistive dividing bridge formed by resistors R2, R3, connected in series between terminals K and A determines, with resistor R1, a coefficient of proportionality between voltage Vref and load supply voltage V.sub.KA. The midpoint of the association of resistors R2, R3, is connected to terminal Vref of regulator 1.
A problem which arises in this type of regulator is related to the current operating range of the regulator, that is, the range of currents in which the regulator is capable of maintaining voltage V.sub.KA substantially constant.
FIG. 3 shows an example of conventional diagram of a regulator 1 such as shown in FIGS. 1 and 2.
Regulator 1 includes a circuit 3 for generating temperature compensated reference voltage Vref, a branching stage 5 taking a current Is from terminal K and a differential stage 4 for regulating current Is according to the current in the load (not shown in FIG. 3).
Circuit 3 is formed of an NPN transistor N1 mounted in series with three resistors R4, R5, R6, between terminals K and A. The collector of transistor N1 is connected to terminal K and its base forms terminal Vref Circuit 3 also includes a differential stage formed of two PNP transistors P2, P3, the emitters of which are connected to terminal K via a source 6 of constant current, and the collectors of which are connected to terminal A via biasing and gain adjusting resistors R7, R8, R9. The respective bases of transistors P2 and P3 are connected to the terminals of resistor R5 which forms the intermediary resistor of the series association of resistors R4, R5, and R6 between the emitter of transistor N1 and terminal A.
The operation of circuit 3 is well known. The difference .DELTA.Vbe between the respective base-emitter voltages Vbe2 and Vbe3 of transistors P2 and P3 is directly proportional to temperature. The voltage across the series association of resistors R4, R5, and R6 (thus, the voltage between the emitter of transistor N1 and terminal A) is also proportional to temperature. Conversely, base-emitter voltage Vbe1 of transistor N1 is inversely proportional to temperature. Thus, voltage Vref is stable in temperature.
The collectors of transistors P2 and P3 are connected to the respective bases of two PNP transistors P4 and P5 of stage 4. The emitters of transistors P4 and P5 are connected to terminal K via a source 7 of constant current. The collectors of transistors P4 and P5 are connected to the respective collectors of NPN transistors N6 and N7, the emitters of which are connected to terminal A and the bases of which are connected to the collector of transistor N6. The collector of transistor N7 forms an output of stage 4 which provides a current I5 for controlling an NPN transistor N8 of stage 5. The collector of transistor N8 is connected, possibly via a Darlington assembly forming a current amplifier 8, to the base of an NPN transistor N9 forming a branching transistor mounted between terminals K and A. The emitters of transistors N8 and N9 are connected to terminal A. The amplifying coefficient (-X) brought in by Darlington assembly 8 has been indicated as negative to symbolize the inversion of the current direction between the base current of transistor N9 and the collector current of transistor N8. The collector of transistor N8 is connected to terminal K through a current source 12.
Assuming that the current surged by the load decreases, current I (FIGS. 1 and 2) arriving on terminal K tends to decrease. As a result, voltage V.sub.KA, and thus voltage Vref, tends to increase. This leads to circulation of a current in the base of transistor N1 and to the circulating of a current It through resistor R5. The voltage difference .DELTA.Vbe between the bases of transistors P3 and P2 becomes positive. The unbalance of the collector currents of transistors P3 and P2 generates a positive voltage difference .DELTA.V between the bases of transistors P5 and P4, which causes an increase of current I5 and, thereby, an increase of current Is which restores the balance of the total current I taken on terminal K to compensate the current surge decrease in the load.
To maintain voltage Vref constant, the unbalance of the currents of the collectors of transistors P2 and P3 must be as low as possible. This requires that value .DELTA.V is the lowest possible, that is, base current I5 of transistor N8 remains negligible with respect to current I4 of current source 7 in the variation range of current Is. Indeed, the higher the ratio between current I5 and current I4 (more precisely, the ratio between current I5 and half current I4 due to its distribution between the two branches of stage 4), the more value .DELTA.V causes an important unbalance between the base currents of transistors P2 and P3 and the more voltage Vref drifts.
The operating range of the regulator is thus linked to the range of current Is in which base current I5 of transistor N8 remains low with respect to current I7.
The role of amplifying assembly 8 is precisely to bring in a gain factor between current I5 and base current Ib9 of transistor N9. The gain of the assembly, linked to the number of Darlington stages that it includes, must however remain limited in order not to generate a stability problem of the control loop caused by too much gain.
As a particular example, the variation of the voltage Vref of a regulator such as shown in FIG. 3, sized for an intrinsic consumption (that is, without taking branching current Is into account) of about 50 .mu.A, is about 330 .mu.V per milliamperes of variation of current Is. The maximum operating current Is of such a regulator, to maintain reference voltage Vref plus or minus one centivolt, is then around 15 mA.
A conventional solution to increase the range of current in which the regulator maintains voltage Vref is to increase the size of transistors forming current source 7, in order to increase, permanently, current I4 of supply of regulation stage 4, and, thus, the collector currents of transistors P4 and P5.
A disadvantage of such a solution is that it increases the intrinsic power consumption of the regulator. As a result, if the upper limit of the current operating range is higher, the minimum operating range of the regulator is also higher.
More generally, the problems discussed hereabove arise for any differential stage having a non-differential output connected at the output of a differential amplifier.